Dual clutch transmissions of this kind are known, for example, from prior publications EP 1 141 580 B1, EP 1 342 934 A2, EP 1 342 935 A2, EP 0 061 845 A2 and DE 43 30 170 A1. Such an automatic transmission has two axial parallel clutch arrangements actuated by a motor shaft, wherein the first clutch on the input side is fitted to a centrical transmission input shaft and the second clutch, next to the gear trains of the transmission, is respectively attached to a coaxial hollow shaft facing the transmission shaft. The transmission input shaft and the hollow shaft are each fitted with fixed gears which engage through respectively related gears on two countershafts of the transmission. In this respect, such automatic transmissions have two so-called drive constants, i.e., gear assemblies that are driven directly by both clutches. One of the two countershafts is embodied as a hollow shaft and is rotationally mounted on the other countershaft. For the purpose of implementing six forward gears and one reverse gear, idler and fixed gears are arranged on both countershafts, which engage with idler and/or fixed gears on a transmission drive shaft which, in turn, is concentrically aligned with both transmissions input shafts.
The particularly known six-gear dual clutch transmission from DE 43 30 170 A1 is additionally characterized in that the gears of the drive constants are embodied as a fixed gear on one input shaft and as an idler gear on the other input shaft. It is further known from this publication that a rotationally rigid connection between the radial inner transmission input shaft and the transmission output shaft has to be created in order to implement a forward gear. Furthermore, it is provided that the countershaft, embodied as a hollow shaft, holds an additional fixed gear which meshes with an idler gear rotationally mounted on the transmission output shaft. This idler gear is connectable with the transmission drive shaft by way of a gear shift sleeve.
Finally, it can be derived from DE 43 30 170 A1 that the state of the art transmission design for the purpose of improving driving comfort aims to provide as many choices as possible in the speed reduction ratio for the individual forward gears of the transmission. In this case, the value of the so-called ratio range is of interest which takes place through the quotient of the speed reduction ratio between a higher and a neighboring lower gear. The ratio range is generally between 1.0 and 2.0. In order to attain high driving comfort, the aim is to set the ratio range digressively from the lowest to the highest gear so that the speed reduction range change becomes less and less from the lowest to the highest gear from one gear relative to the other.
Furthermore, it is in the interest of the vehicle manufacturer to optimally transmit the input power of the prime mover of a vehicle at the lowest possible consumption in every driving mode to all drive gears. This requires that the speed reduction ratio values of the individual gears be spaced apart as far as possible so that there is a wide so-called overall gear ratio. Particularly in vehicle drive trains with a diesel-internal combustion engine, a comparably high overall gear ratio is desirable, which should show a value in excess of 6.0. An overall gear ratio value of 6.0, therefore, means that the speed reduction ratio in the first transmission gear is six times higher than the speed reduction ratio in the highest transmission gear.
On the other hand, there is a need for multi-step automatic transmissions, for example, in sports cars with 4-stroke-engines with a comparable low overall gear ratio, which can certainly have a value of below 5.0. Based on the prior art, this requires a substantially different automatic transmission design which means that a transmission manufacturer must provide additional means of production, as well as maintain a high level of inventory for spare part demands.
Against this background, it is the object of the present invention to provide a dual clutch transmission with more than six forward gears which can be manufactured, through minimal variations of the transmission component, with different speed reduction ratios, overall gear ratios, direct gears and overdrive gears.
The object is achieved with the characteristics of the main claim. Advantageous developments and embodiments of the invention can be derived from the dependent claims.